The present invention relates to a fluid separation apparatus utilizing a membrane to effect the separation. The fluid separation may be a (1) separation of gaseous mixtures by permselective gaseous diffusion; (2) separation of dissolved solids from liquids by reverse osmosis; (3) separation or concentration of liquids from mixtures of liquids and dissolved solids by direct osmosis; (4) separation or concentration of liquids and solids by dialysis; (5) separation of liquids from mixtures of liquids by reverse osmosis or ultrafiltration; (6) the separation of solids from liquids by ultrafiltration; (7) gaseous exchange between liquids; and (8) exchange between a gas phase and a liquid phase. This invention particularly relates to a stacked assembly of membrane and spacer layers in a simplified and commercially advantageous plate and frame arrangement and to the method of manufacturing such an apparatus.
Fluid separations utilizing membranes offer many advantages over conventional separation techniques. In the field of water desalination, reverse osmosis membrane processes are now dominating this field, reducing the reliance on ion exchange, distillation and electrodialysis. In gas separation, membrane processes are being accepted as a new method to remove components from a gaseous mixture, such as the removal and recovery of hydrogen from process gas streams and the removal of carbon dioxide from natural gas. Ultrafiltration membrane processes are becoming very important in water treatment, pharmaceuticals processing, food processing, wastewater cleanup, and materials recovery from fluids.
The various types of fluid separations discussed above are not of a simple filtration type, but involve the feed fluid being contacted onto one side of the membrane and one or more components of the feed fluid passing through the membrane leaving the feed fluid depleted in those components. The feed fluid continually sweeps the surface of the membrane, carrying away the nonpermeating components in the feed fluid. After the feed fluid has contacted the membrane it becomes the residue or concentrate. The residue fluid may be the important component of the system or it may be a waste component.
There are many types of membranes in commercial use and many more in development throughout the world. One thing that all membranes share, regardless of their application, is that they must be supported and housed in a suitable package. If a membrane is to effect its intended purpose, that is to separate some component from a mixture, the membrane package apparatus must:
1. Support the membrane against the static and dynamic pressures and forces applied to one side of the membrane. PA0 2. Provide a passage to the membrane for the feed fluid. PA0 3. Provide for the removal of the fluid that permeated the membrane. PA0 4. Prevent the contamination or mixing of the feed fluid and the fluid permeating through the membrane. PA0 5. Provide a passage for the residual fluid after it has contacted the membrane. PA0 6. Provide a safe housing to contain the fluids being separated. PA0 7. Be economical to manufacture.
The method of assembling the membrane into the package must be efficient to allow the greatest amount of membrane inside the package as possible. The assembled package should also be compact utilizing the least amount of space practical.
As membranes were developed, methods to package them were required for the membranes to function. The first membrane packages were of the plate and frame type, borrowed and modified from the filtration industry, U.S. Pat. Nos. 3,473,668 and 3,209,915. These devices consisted of top and bottom plates with supporting frames and plates in between, U.S. Pat. No. 2,597,907. Feed fluid is introduced through the top plate and passes down to the feed fluid spacers by way of a manifold on one side of the plate and frame assembly. The component of the feed fluid that permeates the membrane enters a permeate carrier layer and is carried out of this layer to the permeate manifold. Complicated porting was required to route the feed fluid through the plate and frame stack to achieve the required recovery rates and to remove the permeate fluid from the stack. The feed fluid depleted in the more permeable component becomes the concentrate or residue and is removed from the assembly via the concentrate manifold. The plate and frame assembly must be constructed of materials that are strong enough to contain the feed fluid under pressure. The early plate and frame assemblies were square or rectangular in shape.
The early attempts to commercialize reverse osmosis and gas separation membranes utilized the plate and frame type of assembly. The process of using membrane for these applications was demonstrated but the economics of the systems were very poor. This was due, not to poor performance of the membrane but, to the very high cost and low packing density of the plate and frame assemblies. Since it was not economical to utilize membrane for either reverse osmosis or gas separation processes with the plate and frame type of membrane assembly, other packaging techniques were investigated and developed.
In 1970 the first commercial high pressure membrane system was installed. This reverse osmosis system utilized cellulose acetate membranes packaged in a spiral wound element configuration, such as is disclosed in U.S. Pat. No. 3,417,870. Flat sheet membranes packaged into spiral wound elements were then joined on the commercial scene by membranes fabricated into hollow fine fiber bundles and into tubular assemblies. Many high pressure industrial uses of synthetic membranes have been commercialized since 1970 utilizing these three main types of membrane packages.
Plate and frame membrane assemblies have been improved during the past 25 years but the devices as currently manufactured are not competitive with membrane packages using hollow fine fibers or spiral wound elements. This is because the plate and frame type of membrane packages are very complicated and therefore expensive to manufacture. The separation devices of Iaconelli U.S. Pat. No. 3,695,444 and Olsen U.S. Pat. No. 3,623,610 are typical of early plate and frame assemblies. Typical of current art is the separation cell of Kraus U.S. Pat. No. 4,340,475. In this device complicated support, separator, collector and distribution plates are required. The membrane supporting plate for example is formed from many pieces and requires a complicated manufacturing process to produce. The separator plates are likewise complicated. The plates are not only expensive to manufacture they are also thick, on the order of 0.1 to 0.25 inches. Thick plates of this type are unable to compete with the hollow fine fibers or spiral wound elements because of their very low membrane packing densities. In U.S. Pat. No. 4,255,263, Galami describes another variation of a stacked separation device. This device also has complicated plates, requiring many parts and materials. In this device the plates are also thick leading to low packing density. In U.S. Pat. No. 4,310,416 Tanaka describes another plate type membrane device. Again, the plates are very complicated and the assembly complex, with many seals and ports that must be connected between the various plates. This device also has thick plates, again leading to low membrane packing density.
In U.S. Pat. No. 4,243,536 Prolss describes a stacked assembly of disk-shaped elements which are located concentrically around a permeate collection pipe and within a pressure vessel. Membrane layers are placed on both sides of a membrane support and permeate carrier layer. This permeate layer is molded from plastic and contains raised squares and fluid conduction passages. This layer is rather thick being about 0.25 inches, and conveys the permeate to the centrally located collection pipe. The centrally located pipe contains ports or holes that are precisely located in the center of the permeate collection layer. Located above and below the permeate carrier-membrane assembly is the feed space which is also thick being about 0.25 inches. The feed fluid enters a pipe located next to and off center from the permeate pipe. Ports or holes are located in the feed fluid pipe that distribute the feed fluid into the feed fluid space. The feed fluid spacer layers must be precisely located to match up with the ports in the feed fluid distribution pipe. At the edge of the disk is a cut-out that contains the residual fluid collection pipe. This collection pipe also contains precisely located holes or parts that must match up with the feed fluid space. The assembly is stacked on the centrally located permeate pipe and placed in a pressure vessel. The present invention improves over the art of Prolss in several ways: first there are no internal pipes; feed fluid, residual fluid, and permeate fluid are conducted within the stacked assembly through channels formed by the registration of the notches cut into the layers of material that make up the stacked assembly. Second, the membrane packing density is much greater than the device of Prolss; the spacer layers are much thinner resulting in much higher packing density. Third, the device is much more economical to manufacture than the device of Prolss; the layers are made from inexpensive matted, knitted or woven materials rather than the injection molded materials of Prolss. Fourth, the assembly of the present stacked assembly is significantly easier than assembling the device of Prolss; there are no internal pipes with precisely drilled holes that the various layers must be aligned to. Fifth, the permeate carrier of the present invention acts as both a membrane support layer and a permeate carrier. This layer can be a paper or paperlike material, a woven material or a knitted material. All of these types of materials can be made to both support the membrane and conduct the permeate to the central permeate collection channel. In the stacked assembly of Prolss, the membrane support and permeate conduction layer are molded from a plastic. The layer has flat, smooth landings that support the membrane and trough-shaped conducting channels molded between the landings. This approach is both uneconomical and inefficient when compared to the present invention.
Conventional fluid separation processes (such as distillation, cryogenic fractionation and physical and chemical solvent extraction) have one advantage over the newer membrane based processes, this being that large scale projects can benefit from economies of scale. World scale size separation plants can use larger vessels, columns and piping, thereby taking advantage of the economies of scale. Membrane based processes, however, have not been able to take advantage of economies of scale because the present membrane packaging techniques are very limited in upward growth in size.
The present spiral wound membrane package is limited to about 12 inches in diameter and 60 inches long, containing approximately 1300 square feet of membrane. A spiral wound membrane element must be constructed of multiple leaves of membrane, permeate carrier and feed fluid spacer layers. A spiral wound membrane element with a diameter of 4 inches and a length of 40 inches has 3 or 4 leaves, each containing 16 to 25 square feet of active membrane area. The leaf length is limited because the efficiency of the permeate carrier material is poor. A leaf length of more than 40 to 60 inches causes a back pressure in the leaf that is too high to be acceptable. An increase in diameter greatly increases the number of individual membrane leaves required in each spiral wound element. A 12 inch diameter spiral wound element contains between 24 and 30 individual leaves. Making spiral elements with larger diameters, such as 16 inches results in almost insurmountable manufacturing problems. There is so much membrane in each element and so many individual membrane leaves to handle that the statistical chance of a defect in each spiral element is very high.
One of the major improvements of the instant invention over previous art is that the path the permeate is required to travel is greatly reduced. In a membrane package of the instant invention made 60 inches in diameter the permeate has only to travel a maximum of 30 inches to reach the central collection channel.
For hollow fine fiber bundle elements the same problem exists. The current hollow fiber elements are 8 to 10 inches in diameter. Increasing the diameter to 12 to 16 inches increases the complexity of manufacture and the likelihood of a defect is very high. The hollow fine fiber bundles are limited in length due to the pressure drop of the permeate in the bore of the fiber. Current designs of hollow fiber bundles are limited to about 12 feet in length.
Even if it were practical to manufacture spiral wound and hollow fiber elements in 16 inch diameters it is not a significant increase in the economies of scale over existing element sizes. To be able to compete in world class projects with conventional technologies, membrane packages and pressure vessels to contain the packages must be increased to the order of 36 to 60 inch diameters.
One of the advantages of the present invention is that it provides a method and apparatus to package membranes for these very large systems. It is just as feasible to an facture this new membrane package in 36 to 60 inch diameters as it is in 8 to 12 inch diameters. A major advantage of the present invention is that each membrane-permeate carrier assembly can be pretested for defects before it is assembled into the pressure vessel or module. This means that only a few square feet of membrane is lost due to a defect in the membrane or adhesive sealant lines.
Concurrent with the development of the high pressure industrial membrane applications was the development of small and low pressure membrane packages. Examples of these packages are artificial kidneys, home and laboratory reverse osmosis systems, food and beverage processing elements and many special membrane packages. These systems utilized spiral wound elements, hollow fiber bundles and tubular elements but also led to the development of many variations of the original plate and and frame package.
The present invention is applicable to these very small membrane packages including small medical units. In this regard, the present invention is particularly adaptable to portable blood oxygenators and similar devices. The high density of the packing in the present invention makes possible devices which are both efficient and compact.
In general, the present invention provides a novel membrane device which is simple and less costly to fabricate, physically compact, and highly efficient in operation. Accordingly, it is to be expected that our invention will be widely adoped in the art.